Strain gauges are useful in various civil, industrial, and medical applications to measure or monitor mechanical strain in bridges and other infrastructure, aerospace components, and medical prostheses, to name a few. Typical strain gauges are made from metal foils that require an interface with other electronic equipment and a power source. Wireless strain sensors that can be remotely accessed are attractive for some applications. An example is in vivo monitoring of medical implants, where a wireless approach is often the only practical way to obtain strain measurements over long periods. Some sensors are powered sensors and require a battery and/or an energy harvesting system to operate. Other sensors are passive sensors that do not require a built-in power source to operate. Passive sensors can be simpler and more robust than their powered counterparts. Various techniques have been investigated for use in passive strain sensing, including the use of inductive coupling, radio frequency (RF) electromagnetic backscattering, surface acoustic waves, and optical methods.
Magnetoelastic materials have been proposed for use in wireless sensing applications to measure mass loading, viscosity, fluid flow, pressure, position, and other parameters. In the presence of a magnetic field, a magnetoelastic material exhibits the Joule magnetostriction effect, in which the magnetic field induces strain in the material. Conversely, when subjected to an applied strain, a magnetoelastic material exhibits the Villari effect, in which the applied strain induces magnetization in the material. Both of these effects include the rotation of magnetic moments within the material. Joule magnetostriction is moment rotation under the applied magnetic field, and the Villari effect is moment rotation under the applied strain. When an alternating magnetic field is applied to a magnetoelastic material, mechanical vibration is induced via the Joule effect and results in strain-induced material magnetization via the Villari effect. The use of magnetoelastic materials in strain sensors has been limited.